The present invention relates to a train of tires intended to equip a vehicle and, more particularly, tires having treads the tread patterns of which are directional, that is to say, tread patterns which are related in their design to the direction of rotation of the tires.
Train of tires as used herein means the tires necessary for the complete equipping of the vehicle.
Directional tread patterns have been known for a long time and have been used for agricultural tires or tires used on building sites or in mines. This tread pattern principle has been employed for several years with a certain amount of success in tires intended to travel at high speed, the combined characteristics of the grooves and of the ribs being selected so as to obtain the best possible compromise between the properties of adherence on the different soils encountered by the tire, the wear and endurance properties, the properties of rolling resistance and of road-holding at high speed, the properties of behavior of the vehicle on a straight line and on curves, and the properties with regard to noise.
It is known that a directional tread, the grooves and ribs of which are inclined significantly with respect to the circumferential direction of the tire, with the ends located at the center of the tread penetrating first into the region of contact between the tire and the ground, assures only a slight generation of noise. This is part of the teaching contained in FR 2 637 547 or in EP Application 0 402 303, which show so-called "V"-shaped or herringbone patterns. Now, it is in fact true that the tread patterns described generate acoustic intensity or acoustic power which is less at the front of the tire/ground contact ellipse when the recommended direction of rotation is respected. This is external noise, that is to say noise perceived outside the vehicle. This outside noise is measured either close to the tire, at the front or rear of the tire/ground contact ellipse, or "on the edge of the track" on which the test vehicle travels.
There are also so-called interior noises which are much more disturbing to the occupants of the vehicle and are greater the more the tread pattern of the tires is formed of elements which are strictly identical on the circumference. The well-known "variable pitch" technique, the circumferential lengths of the tread elements being different and determined in such a manner that the acoustic energy is distributed over a wide frequency spectrum in order to avoid resonances, permits a substantial reduction in the internal noises. As the latter are particularly influenced by the size of the tire, that is to say by the ratio of the height of the tire to the maximum axial width of the tire, by the speed of travel of the vehicle equipped, and by the location of the transverse grooves, they must be further reduced.
The studies carried out by the applicant on the sources of the noises given off by the travel of the tires and as to the manner in which these noises are transferred to the occupants of a vehicle have made it possible to obtain an improvement without increase in the level of the acoustic power given off to the outside.